Some 650,000 homes were damaged by floods in China

Senior UN official Klaus Toepfer said climate change was a reality that would increasingly lead to human suffering and economic hardship.

Natural disasters, mostly caused by extreme weather, cost more than $60bn this year alone, the international conference in Italy was told.

However a senior US politician has cast doubt on the climate change warnings.

Organisers of the conference in Milan had hoped to get final ratification needed to put the 1997 Kyoto Protocol on greenhouse gas emissions into effect.

But in the last few days, Russia - whose support is vital after the US pulled out of the accord - has said it is having second thoughts about signing.

Extreme weather

The conference - into its 10th day before finishing on Friday - heard that the effects of climate change were already being felt.

"Climate change is already having an impact on mankind, especially in developing countries," said China's chief delegate Liu Jiang.

Global warming is the greatest hoax ever perpetrated on the American people and the world US Senator James Inhofe

Mr Toepfer said: "Climate change is not a prognosis, it is a reality that is and will increasingly bring human suffering and economic hardship".

UN Secretary-General Kofi Annan spoke of the heightened frequency of extreme weather events in recent years.

""There is growing concern that this trend is likely to continue," he said.

The conference was shown some of the findings by insurance firm Munich Re, which has been tracking the cost of global natural disasters.

Europe's extreme summer heat wave was the biggest single event this year - costing more than $10bn in agricultural losses alone and killing some 20,000 people.

However, some remain unconvinced about the arguments for accords such as Kyoto.

'Hoax'

US Senator James Inhofe, who chairs the Senate Committee on Environment and Public Works, says it is inconsistent with freedom, prosperity and environmental policy progress.

"I'm becoming more and more convinced... that global warming is the greatest hoax ever perpetrated on the American people and the world," he told a conference briefing.

Financial cost of 2003's disasters Europe's summer heat wave - $10bn Flooding in China - nearly $8bn Tornado in US Midwest - $3bn insured losses Source: Munich Re

The BBC's David Bamford in Washington says Mr Inhofe's view fits neatly with the majority view in the US congress - that America should do nothing about the issue.

Our correspondent says members of the Republican-led congress know the US is at odds with scientific experts.

However, obliging industrial plants to reduce emissions would be a vote loser, because most Americans would assume it meant a reduction in production, job losses and a rise in household energy bills.

The last attempt in October to introduce such a bill failed in the Senate, even though it was co-sponsored across party lines by Democrat Joe Liebermann and Republican John McCain.